The present invention relates to an optical transmitter for use in optical communication systems and, more particularly, to an optical transmitter provided with an external optical modulator whereby wavelength chirping is prevented.
Conventionally, a direct modulation system, in which the current flowing through a laser diode is modulated with a data signal, has been in use as the optical transmitter in an optical communication system. In the direct modulation system, however, the effect of wavelength variation (chirping) of the optical signal output becomes greater as the transmission speed increases and hence long-distance transmission becomes difficult on account of wavelength dispersion within an optical fiber.
Therefore, optical transmitters have been studied which are provided with an optical modulator such as a Mach-Zehnder modulator, which produces little chirping from its principle. In order to achieve stabilized operations for a long time in an optical transmitter provided with such an optical modulator, it becomes necessary to keep the optical signal output therefrom stabilized under conditions of varying temperature and changing properties by aging.
A conventional optical transmitter provided with an optical modulator includes, for example, a light source formed of a laser diode or the like, an optical modulator for modulating the emitted light from the light source according to a driving voltage, thereby converting an input signal into an optical signal, and a drive circuit for generating the driving voltage according to the input signal.
In such an optical transmitter, a method has been proposed to compensate for deviation of the operating point of the optical modulator due to temperature variation and the like. In this method the modulator is driven with a signal having a low-frequency signal superposed on one side of the logic levels of the input signal, and the operating point is controlled according to the phase of the low-frequency signal detected from the optical signal output of the optical modulator (Japanese Laid-open Patent No. 42365/1974).
When using this method, however, the control of the optical modulator for keeping it at its optimum operating point has been difficult. This difficulty has been due to the occurrence of changes in the mark-space ratio of the input signal and increase in the rise time and fall time thereof. If the optimum operating point is not maintained, the waveform of the optical signal becomes deteriorated. Therefore, it is desired that a means be provided that does not rely on the input signal, of preventing the waveform of the output optical signal from being deteriorated due to a drift of the operating point.
Incidentally, it is confirmed that a wavelength variation is produced even in the Mach-Zehnder optical modulator and a waveform change is caused by wavelength dispersion in an optical fiber (T. Okiyama et al., "10 Gb/s Transmission in Large-Dispersion Fiber Using a Ti: LiNb03 Mach-Zehnder Modulator", I00C '89, Vol. 3, pp. 208-209). Since such wavelength variation causes improvement or deterioration in the waveform depending on the sign of the wavelength dispersion in the optical fiber, it is desired that an optical transmitter be provided which prevents the occurrence of such waveform deterioration irrespective of the sign of the wavelength dispersion be provided.